1. Field of the Invention
The present invention relates generally to vacuum transport belt apparatus, such as useful in ink-jet hard copy apparatus and methods of operation and, even more specifically, to a restricted flow vacuum system providing media cockle control and having minimal airflow-induced ink drop trajectory effects.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W.J. Lloyd and H.T. Taub in OUTPUT HARDCOPY [sic] DEVICES, chapter 13 (Ed. R.C. Durbeck and S. Sherr, Academic Press, San Diego, 1988). As providing background information, the foregoing documents are incorporated herein by reference. Further details of basic inkjet printing technology are also set forth below in the Detailed Description of the present invention with respect to FIG. 1.
It is known to use a vacuum induced force to adhere a sheet of flexible material to a surface, for example, transporting sheet metal, holding a sheet of print media temporarily to a transport system or platen, and the like. Hereinafter, "vacuum induced force" is also referred to as "vacuum induced flow," "vacuum flow," or more simply as just "airflow," "vacuum" or "suction," as best fits the context. Such vacuum holddown systems are a relatively common, economical technology to implement commercially and, in printing technology, can improve hard copy apparatus throughput specifications. For example, it is known to provide a rotating drum with holes through the surface wherein a vacuum type airflow through the chamber formed by the drum cylinder provides a suction force at the holes in the drum surface (see e.g., U.S. Pat. No. 4,237,466 for a PAPER TRANSPORT SYSTEM FOR AN INK JET PRINTER (Scranton) or U.S. Pat. No. 5,081,506 for a TRANSFER SYSTEM FOR A COLOR PRINTER (Borostyan)). The term "drum" as used hereinafter is intended to be synonymous with any curvilinear implementation incorporating the present invention; while the term "platen" can be defined as a flat holding surface, in hard copy technology it is also used for curvilinear surfaces, e.g., the ubiquitous typewriter rubber roller; thus, for the purposes of the present application, "platen" is used generically for any shape paper holddown surface--stationary or movable--as used in a hard copy apparatus. Permeable belts traversing a vacuum inducing support have been similarly employed (see e.g., Scranton and U.S. Pat. Appl. Ser. No. 09/ 163,098 by Rasmussen et al. for a BELT DRIVEN MEDIA HANDLING SYSTEM WITH FEEDBACK CONTROL FOR IMPROVING MEDIA ADVANCE ACCURACY (assigned to the common assignee of the present invention and incorporated herein by reference)).
Generally in a hard copy apparatus implementation, the vacuum device is used either to support cut-sheet print media during transport to and from a printing station (also known as the "print zone" or "printing zone") of a hard copy apparatus, to hold the sheet media at the printing station while images or alphanumeric text are formed, or both. In order to further simplify description of the technology and invention, the term "paper" is used hereinafter to refer to all types of print media and the term "printer" to refer to all types of hard copy apparatus; no limitation on the scope of the invention is intended nor should any be implied.
In essence, the ink-jet printing process involves digitized, dot-matrix manipulation of drops of ink, or other liquid colorant, ejected from a pen onto an adjacent paper. One or more ink-jet type writing instruments (also referred to in the art as an "inkjet pen" or "print cartridge") include a printhead which generally consists of drop generator mechanisms and a number of columns of ink drop firing nozzles. Each column or selected subset of nozzles (referred to in the art as a "primitive") selectively fires ink droplets (typically each being only a few picoliters in liquid volume) that are used to create a predetermined print matrix of dots on the adjacently positioned paper as the pen is scanned across the media. A given nozzle of the printhead is used to address a given matrix column print position on the paper (referred to as a picture element, or "pixel."). Horizontal positions, matrix pixel rows, on the paper are addressed by repeatedly firing a given nozzle at matrix row print positions as the pen is scanned. Thus, a single sweep scan of the pen across the paper can print a swath of dots. The paper is stepped to permit a series of contiguous swaths. Dot matrix manipulation is used to form alphanumeric characters, graphical images, and even photographic reproductions from the ink drops. Page-wide ink-jet printheads are also contemplated and are adaptable to the present invention.
A well-known phenomenon of wet-colorant printing is "paper cockle," the irregular surface produced in paper by the saturation and drying of ink deposits on the fibrous medium. As a sheet of paper gets saturated with ink, the paper grows and buckles in a seemingly random manner. Paper printed with images are more saturated with colorant than simple text pages and thus exhibit great paper cockle. Colors formed by mixing combinations of other color ink drops form greater localized saturation areas and also exhibit greater cockle tendencies.
As the ink-jet writing instruments--often scanning at a relatively high rate across the paper--expel minute droplets of ink onto adjacently positioned print media and sophisticated, computerized, dot matrix manipulation is used to render text and form graphic images, the flight trajectory of each drop is critical to print quality. Printing errors (also referred to in the art as "artifacts") are induced or exacerbated by any airflow in the printing zone. Thus, use of a vacuum platen and vacuum transport device in the printing zone of an ink-jet printer creates an added difficulty for the system designer. One solution to the problem is set out in applicants' pending application 09/514,830, filed on Feb. 28, 2000, for a LOW FLOW VACUUM PLATEN FOR AN INK-JET HARD COPY APPARATUS. In essence, it employs a platen having an array of vacuum ports that are each filtered. The filter is constructed to provide restricted airflow such that media holddown pressure remains substantially uniform when the platen is either fully covered or partially uncovered. The filter mechanism provides airflow restrictions such that ink drop flight trajectories in the printing zone are unaffected, acoustic dampening of the vacuum pump is provided, and vacuum pressure is kept relatively high at the print media edges.
There is still a need for a commercial, low-cost, vacuum system for use in an ink-jet printing zone which will assist in minimizing cockle and provide a minimal airflow impact on ink-jet drop flight trajectory.